Accurate power sensing circuit for power amplifiers

ABSTRACT

An amplifier circuit for amplifying radio frequency signals having temperature compensation and bias compensation includes a radio frequency power amplifier that receives an input radio frequency signal and outputs an amplified radio frequency signal, and a first transistor performing as a detector diode with its collector and base connected. The base of the first transistor receives the amplified radio frequency signal from the power amplifier, a second DC bias input signal from a regulated DC source, and a third power-sensing signal. The amplifier circuit further includes a second transistor to amplify the DC component of the RF signal from the base of the first transistor. The base of the second transistor is coupled to the base of the first transistor. The collector of the second transistor outputs the power-sensing signal, which is coupled to the regulated DC source through a resistor.

This application claims priority to Provisional Application Ser. No.60/397,261, filed on Jul. 19, 2002, titled “Power Amplifier Modules forWireless LAN Applications”, the content of which is hereby incorporatedby reference.

RELATED APPLICATION

The present invention is related to the commonly assigned U.S. patentapplication Ser. No. 10/041,863, filed on Oct. 22, 2001, titled“Multilayer RF Amplifier Module”, by Wang, et al., and the commonlyassigned and concurrently filed U.S. patent application “Power AmplifierModule for wireless communication devices” by Ichitsubo et al. Thedisclosures of these related applications are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to radio frequency (RF) poweramplifiers, more specifically, to power sensing for RF power amplifiers.

BACKGROUND

Radio frequency transmission of an electrical signal occurs in a broadfrequency spectrum from several megahertz (MHz) to tens of gigahertz(GHz). Common RF transmission systems include an antenna that transmitsand receives RF signals and a low noise amplifier that amplifies aninput RF signal from the antenna and a power amplifier to generate anamplified signal to be delivered to the antenna. The power of the outputsignals from the power amplifiers are required to be controlled withinstringent specifications as well as regulatory standards, to assure thequality of the RF transmission signals. Moreover, many portable wirelesssystems are required to increase or reduce the transmitted powerdepending upon signal strength, transmission range, the types of digitalsignal modulation such as Quadrature Phase Shift Keying (QPSK) andOrthogonal Frequency Division Multiplexing (OFDM), as well assurrounding interfering signals. These requirements can be met by apower sensing circuit that samples the output power of the poweramplifier and outputs a power sensing signal for power control. However,variations in power sensing signals due to temperature and DC bias arealso passed on to the power output through the control loop. Variationsin power sensing signals can significantly affect the qualities of theoutput RF signals. A need therefore exists for accurate and reliabletechniques for power sensing for RF power amplifiers with goodtemperature and DC bias compensations.

SUMMARY

The invention system generally includes a power sensing circuit forsensing radio frequency signals, comprising:

a) a first transistor to perform as a detector diode, including

i) a first base to receive a first radio frequency input signal output,a second DC bias signal from a regulated DC source coupled through aresistor, and a third power-sensing signal; and

ii) a first collector connected to the first base;

b) a second transistor to amplify the DC component of the RF signal fromthe first base, including

i) a second base connected to the first base through a low-pass filter;and

ii) a second collector to output the power-sensing signal to be coupledto the regulated DC source through a resistor, said power-sensing signalbeing substantially proportional to the strength of the first radiofrequency input signal; and

c) an output port coupled to the second collector through a low-passfilter, to output the power-sensing signal.

In one aspect, the present invention provides a power sensing circuitfor detecting power of a power amplifier. The power sensing circuitincludes a detector diode using a transistor and a DC amplifier using asecond transistor. The second transistor acts as a current mirrorcircuit regarding the DC current component of the first transistor andcompensates variations in the power sensing circuit. The power sensingsignal is provided in a single output terminal.

In another aspect, the present invention provides a circuit design thatis simple and easy to implement using Heterojunctioh Bipolar Transistors(HBT) materials such as InGaP/Ga As which improves reliability relativeto prior art.

An advantage of the present invention is that the temperature variationof the power sensing circuit is properly compensated to provide accuratepower sensing at a wide temperature range.

Another advantage of the present invention is that the invention powersensing circuit directly senses the final output RF signal and can thusinclusively detect variations in the whole power amplifying circuit.

Yet another advantage of the present invention is that the inventionpower sensing circuit is simpler and easier to implement compared toprior art systems.

The details of one or more embodiments are set forth in the accompanyingdrawing and in the description below. Other features, objects, andadvantages of the invention will become apparent from the descriptionand drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 is a simplified schematic diagram illustrating a RF power sensingcircuit in the prior art.

FIG. 2 is a schematic circuit diagram illustrating a RF amplifiercircuit having power sensing in accordance to the present invention.

DESCRIPTION OF INVENTION

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the present invention.

FIG. 1 shows a simplified schematic diagram of a prior art system 100disclosed in U.S. Pat. No. 6,265,943. A RF signal is received by theprior art system 100 at the RF-in signal port 102 where it is coupled toa RF amplifier transistor 104 via coupling capacitor 106. An outputmatching network 108 provides impedance matching to the antenna toprovide efficient transmission of the amplified RF signal to theassociated antenna. A small RF sampling transistor 112 is deployed inparallel with the RF amplifier transistor 104, which is physically muchlarger than the small sampling transistor 112. The small samplingtransistor 112 is DC biased via a dedicated bias network 114. The sizeof the small sampling transistor 112 may, for example, be about{fraction (1/250)}_(th) the size of the RF amplifier transistor 104.

The prior art system 100 includes a second small sampling transistor 116to compensate for a dominating quiescent bias current at low powerlevels. This second small sampling transistor 116 is also physicallymuch smaller than the RF amplifier transistor 104 and is optionally thesame physical size as the first small sampling transistor 112. Thesecond small RF sampling transistor 116 is deployed in parallel with thefirst small sampling transistor 112, however, it does not receive any ofthe RF drive energy from the input stage. Rather, the second smallsampling transistor 116 amplifies and receives only the bias currentfrom the bias networking 114. The predicted power can be corrected forbias current effects and bias current shifts. A differential amplifier118 is connected between the PWR_Sense and PWR_Sense_Reference in orderto cancel erroneous contributions of the bias current to the predictedpower.

Illustrated in FIG. 2 is a schematic circuit diagram of a RF amplifiercircuit 200 in accordance to the present invention. The RF amplifiercircuit 200 includes a RF power amplifier 210, a RF output impedancematching circuit 220 and RF power output port 230. The RF amplifiercircuit 200 also includes a RF power sensing circuit 225 that senses theamplified RF signal output by the RF power amplifier 210 and through theRF output impedance matching circuit 220. The RF power sensing circuit225 includes a first transistor 250 that detects RF signal from theoutput of RF power amplifier 210 through a coupling capacitor 240. Thebase and the collector of the first transistor 250 are electricallyconnected so that the first transistor 250 acts as a diode. The currentflowing into the first transistor 250 (Detector) depends on the outputRF voltage of the RF power amplifier 210. The RF power sensing circuit225 also includes a second transistor 270 having its base connected tothe base of the first transistor 250 through a low-pass filter 260 (R3,C3). The second transistor 270 thus functions as a DC amplifier of theDC bias diode (i.e. the first transistor 250).

The collector of the second transistor 270 is coupled to a regulated DCsource 280 by resistor R2. The base and the collector of the firsttransistor 250 are coupled to a regulated DC source 280 by resistor R1.The regulated DC source 280 provides a temperature-compensated, constantvoltage well known in the art.

In this circuit layout, the second transistor 270 acts as a currentmirror circuit regarding the DC current component of the firsttransistor 250. That is:

I ₂ =S×I ₁,

wherein S is the size ratio of the second transistor 270 to the firsttransistor 250, and I₁ and I₂ are the currents flowing through the basesof the first transistor 250 and the second transistor 270 respectively.S=1, if the sizes of the two transistors are the same.

The output RF voltage of the RF power amplifier 210 can be picked up atthe collector voltage of the second transistor 270. The output signal ofthe RF power sensing circuit 225 is coupled to the collector of thesecond transistor 270 through a low-pass filter (R4, C4) that acts as abuffer to isolate the RF power sensing circuit 225 from external RFsignals. The output signal of the RF power sensing circuit 225 issubstantially proportional to the strength of the amplified radiofrequency signal output by power amplifier 210.

The RF amplifier circuit 200 and the RF power sensing circuit 225illustrated in FIG. 2 are simple and easy to implement in HeterojunctiohBipolar Transistor (HBT) Integrated Circuits, for example, using GalliumArsenide materials. The detector diode (i.e. the first transistor 250)and the DC amplifier (i.e. the second transistor 270) are proximity witheach other. The first transistor 250 and the second transistor 270therefore experience correlative temperature variations (thus similarvariations in I-V response curves). For example, an upward temperaturefluctuation causes the I₁ to rise and first base voltage to drop, whichalso causes second base voltage to drop and I₂ to drop. The counterbalance between the two effects effectively compensates temperaturevariations.

Several advantageous distinctions can be found in the RF amplifiercircuit 200 and the RF power sensing circuit 225 in the presentinvention, in comparison to the prior art system 100. One importantdesign difference is that the invention RF power sensing circuit 225 inFIG. 2 directly senses the output signal from the power amplifier 210and the RF output impedance matching circuit 220 whereas the prior artsystem 100 senses the input signal to RF amplifier transistor 104. Thevariations that the invention RF power sensing circuit 225 sensesincludes variations arisen from the RF power amplifier 210 and theoutput impedance matching circuit 220. In contrast, the prior art system100 does not detect and thus cannot compensate variations in the RFamplifier transistor 104 and/or the output matching network 108.Variations in the RF amplifier transistor and output matching circuitcan be caused, for example, by temperature variations, batch-to-batchvariability in component fabrication, and operation variability over acomponent's life cycle, etc.

Another advantage of the present invention is that the design of asingle output terminal in FIG. 2 is easier to use compared to the priorart system 100 in FIG. 2. The first and the second transistors 250, 270are in close proximity to each other and thus experience correlativetemperature variations. The temperature and bias variations within RFpower sensing circuit 225 are inherently compensated in the currentmirror circuit of the two transistors. In contrast, the prior art system100 includes parallel transistors and multiple terminals. The prior artsystem 100 also relies on an additional differential amplifier to cancelerroneous contributions of the bias current.

Although specific embodiments of the present invention have beenillustrated in the accompanying drawings and described in the foregoingdetailed description, it will be understood that the invention is notlimited to the particular embodiments described herein, but is capableof numerous rearrangements, modifications, and substitutions withoutdeparting from the scope of the invention. The following claims areintended to encompass all such modifications.

What is claimed is:
 1. An amplifier circuit for amplifying radiofrequency signals having temperature compensation and bias compensation,comprising: a) a radio frequency power amplifier to receive an inputradio frequency signal and to output an amplified radio frequencysignal; b) a first transistor to perform as a detector diode, includingi) a first base to receive the amplified radio frequency signal, asecond DC bias input signal from a regulated DC source, and a thirdpower-sensing signal; and ii) a first collector connected to the firstbase; and c) a second transistor to amplify the DC component of the RFsignal from the first base, including i) a second base coupled to thefirst base; and ii) a second collector to output the power-sensingsignal, said power-sensing signal being coupled to the regulated DCsource through a resistor.
 2. The amplifier circuit of claim 1 whereinthe power-sensing signal is substantially proportional to the strengthof the amplified radio frequency signal.
 3. The amplifier circuit ofclaim 1 wherein the power-sensing signal is provided to an externalcircuit through a low-pass filter.
 4. The amplifier circuit of claim 1wherein the regulated DC source is coupled to the first base of thefirst transistor through a resistor.
 5. The amplifier circuit of claim 1wherein the first base of the first transistor is coupled to the secondbase of the second transistor through a low-pass filter.
 6. Theamplifier circuit of claim 1 wherein the output of the radio frequencypower amplifier is coupled to the first base of the first transistorthrough a capacitor.
 7. The amplifier circuit of claim 1 wherein theoutput of the radio frequency power amplifier is connected to animpedance-matching circuit which in turn is coupled to the first base ofthe first transistor.
 8. The amplifier circuit of claim 1 wherein thesize of the first transistor is the same as the size of the secondtransistor.
 9. The amplifier circuit of claim 1 wherein at least one ofthe radio frequency power amplifier, the first transistor, and secondtransistor is fabricated with a Heterojunction Bipolar Transistor (HBT).10. The amplifier circuit of claim 1 wherein the input radio frequencysignal is modulated at a frequency in the range of 1 MHz to 10 GHz. 11.A power sensing circuit for sensing radio frequency signals, comprising:a) a first transistor to perform as a detector diode, including i) afirst base to receive a first radio frequency input signal output, asecond DC bias input signal from a regulated DC source, and a thirdpower-sensing signal; and ii) a first collector connected to the firstbase; and b) a second transistor to amplify the DC component of the RFsignal from the first base, including i) a second base coupled to thefirst base; and ii) a second collector to output the power-sensingsignal, said power-sensing signal being coupled to the regulated DCsource through a resistor.
 12. The power sensing circuit of claim 11wherein the power-sensing signal is substantially proportional to thestrength of the first radio frequency signal.
 13. The power sensingcircuit of claim 11 wherein the power-sensing signal is provided to anexternal circuit through a low-pass filter.
 14. The power sensingcircuit of claim 11 wherein the regulated DC source is coupled to thefirst base of the first transistor through a resistor.
 15. The powersensing circuit of claim 11 wherein the first base of the firsttransistor is coupled to the second base of the second transistorthrough a low-pass filter.
 16. The power sensing circuit of claim 11wherein the size of the first transistor is the same as the size of thesecond transistor.
 17. A power sensing circuit for sensing radiofrequency signals, comprising: a) a first transistor to perform as adetector diode, including i) a first base to receive a first radiofrequency input signal output, a second DC bias signal from a regulatedDC source coupled through a resistor, and a third power-sensing signal;and ii) a first collector connected to the first base; b) a secondtransistor to amplify the DC component of the RF signal from the firstbase, including i) a second base connected to the first base through alow-pass filter; and ii) a second collector to output the power-sensingsignal to be coupled to the regulated DC source through a resistor, saidpower-sensing signal being substantially proportional to the strength ofthe first radio frequency input signal; and c) an output port coupled tothe second collector through a low-pass filter, to output thepower-sensing signal.